Flowmeters are known. Many of these devices have been provided with functions to determine further parameters in addition to an actual flow rate measurement. One example can be gathered from the laid-open specification DE 10 118 002. In this document, an AC signal is additionally applied to at least one of the electrodes in a corresponding device and the impedance is measured at the electrode to diagnose the device. This means that the signal which has been fed in and the resultant response function, which is determined in the form of an impedance, are ultimately measured at the same electrode(s).
Further examples are disclosed in DE 196 37716.
In addition, there are magnetic flowmeters which are able to measure a flow rate even in the case of partially filled tubes. For this purpose, in addition to the actual flow rate measurement which constitutes a velocity measurement, the filling level of the medium is additionally determined and is used to determine a volumetric flow rate. A multiplicity of measuring techniques can be used to determine the filling level. It is known in practice to use a separate capacitive measuring method for this purpose. In contrast, the filling level can be determined using an inhomogeneous magnetic field. It is also known practice to determine the filling level by feeding in a current between two electrodes and forming a voltage ratio at two respective further electrode pairs.
In this case, the stated method assumes that at least six electrodes or three electrode pairs are in contact with the medium. If this is no longer the case with low filling levels, the voltage ratio can, in principle, also be formed between two electrode pairs, the current being simultaneously fed in at an electrode pair. A high degree of sensitivity of the device is desired for sensitive measurement of the flow rate and, if appropriate, the detection of disturbing effects. The fact that signals which are fed in and output are measured at the same electrode(s) in this known case results in an incorrect current or voltage measurement. The device thus loses some of its metrological sensitivity.
In addition, a desire to also determine, if appropriate, parameters which only indirectly affect the measurement of the flow rate, such as electrical conductivity of the medium, electrode deposits, etc., in addition to the flow rate has in the meantime been imposed on flowmeters. Therefore, it is of interest not only to be able to determine a filling level by a measurement but also to be able to detect or correct further parameters which may distort the measurement. In this case too, it is desirable to avoid a measurement at an electrode loaded by the incorrect current or voltage measurement in order to be able to carry out such accompanying diagnosis at all.